1. Field of the Invention
This invention relates to an organosilicone-treated silica, a composition containing it, and a process for preparing the composition.
2. Description of the Prior art
For the purposes of regulating fluidability, and reinforcing mechanical strength or improving release property, adhesive property, compression set, heat resistance or chemical resistance, a silica powder is added to an organosilicone composition such as silicone rubbers, silicone greases and silicone oil compounds. Such silica powders include fumed silicas produced by hydrolysis of a silicon compound in an oxygen-hydrogen flame, and wet-process silicas produced by hydrolyzing sodium silicate or the like in water, followed by neutralization. However, these silica powders have a number of silanol groups on their surface; therefore, when mixed into an organopolysiloxane compound as they are, followed by kneading, the silica powders may cause pseudo-crosslinking called structuring or crepe hardening during storage, resulting in serious lowering of fluidability and consistency of the compositions. In particular, in the case of rubber compositions, serious crepe hardening may take place.
As a silica powder which solved the above drawback of the prior art, there are known silicas treated with a chlorosilane of the formula: R.sup.4.sub.3 SiCl, R.sup.4.sub.3 SiCl.sub.2 or R.sup.4 SiCl.sub.3 where R.sup.4 is a hydrocarbon group having from 1 to 8 carbon atoms, or a silazane compound of the formula: (R.sup.4.sub.3 Si).sub.2 NH wherein R.sup.4 is as defined above, thereby the silanol groups on the surface of the silica powders being silylated (See U.S. Pat. No. 3,532,664).
As a process of preparing an organopolysiloxane composition containing the treated silica, there is known a process in which a silica powder is mixed with a silicone rubber, and then the mixture is admixed with said silazane compound, a dialkylsilane diol, an alkoxysilane or the like, followed by reaction, and the silanol groups contained on the surface of the silica powder are thereby silylated in the course of the mixing step (See Japanese Patent Publication (KOKOKU) Nos. 58-8703(1983) and 56-34227(1981)).
In the organopolysiloxane compositions containing a treated silica whose surfaces have been silylated like the above, structuring during storage is reduced.
When a silica powder is treated as described above, however, there is a problem that the chlorosilane, the silazane or the like is reacted with the silica powder to by-produce HCl, NH.sub.3, alcohol, etc. The removal of the by-product may require much time and energy. If the by-product remains in the resulting treated silica, the compositions prepared by adding the treated silica to a silicone oil or silicone gum are poor in heat resistance, durability of release property, durability of transparency, electric insulating properties, etc.
There is proposed a method of silylating silanol groups on the surfaces of a silica powder, using a silylketene acetal having the formula: ##STR2## wherein R.sup.5 is a monovalent hydrocarbon group (Japanese Pre-examination Patent Publication (KOKAI) No. 2-59417 (1990) published on Feb. 28, 1990. This method has the disadvantage that an isobutyrate with disagreeable smell is by-produced.